I get old military ammo boxes and make sure the rubber seal is in good condition. I keep it in good condition with plain old corn oil.

Then i put powder bottles inside and add a few silica packets. Any powder seems to last. Using BH209 it may be a bit extreme but the piece of mind is worth it.

BH209 has actually been tested left open for months indoors and worked fine according to TB. Ive seen it soaked in water then dried and it still fired but i would only do that with EXTREME CAUTION and under very controlled conditions. The water removed some of the added bulk material and the powder weighed less afterwards.

Disclaimer:
I AM IN NO WAY SUGGESTING ANYONE TRIES THIS!!